1. Field of the Invention
This invention relates to an apparatus for producing a thin layer of material on a substrate by laser ablation, and more particularly, to a laser ablation apparatus that includes an optical fiber delivery system to deliver a visible light beam at a high repetition rate.
2. Description of Related Art
A variety of methods have been used to produce thin films of material on substrates. These include ion beam deposition, chemical vapor deposition, plasma enhanced chemical vapor deposition and sputter deposition. Ion beam techniques typically involve producing ions by heating a filament and accelerating electrons through a gas to produce ions that are accelerated towards a substrate in a high vacuum environment. Ion beam systems use differential pumping and mass separation techniques to reduce the level of impurities in the influence to the growing film. Films produced by this method are expensive, limited in variety and have very slow growth rates.
The chemical vapor deposition and plasma enhanced chemical vapor deposition methods are similar in operation and associated problems. Both methods produce collateral products of dissociation that frequently contaminate the growing film. Films produced by these methods have characteristics such as columnar grains and occasional bare spots (holidays) that often make them unsuitable for some of the more demanding commercial uses. These techniques often involve toxic precursors and/or by-products. Additionally, the range of possible products is limited by the availability of suitable gaseous precursors.
Sputtering deposition usually includes one or more ion sources. The poor vacuum and relatively high pressure in sputtering deposition is cumbersome and tends to introduce contamination of the film on a level comparable to those encountered in chemical vapor deposition and plasma enhanced chemical vapor deposition. Because not all elements are sputtered at the same rate, it is often difficult to maintain or control the exact chemical composition of the product material using this technique. This is especially true of complex compounds involving oxides or nitrides.
Pulsed laser deposition of thin films has been demonstrated to be a useful technique for producing a wide variety of thin films on a substrate. Examples include, but are not limited to, high density hard disk coatings, optical coatings, high temperature radiation resistant semiconductors, cutting tool coatings, tribological coatings, heat sinks, field emitters for flat panel displays (FPD's), infrared detectors, low resistivity interconnects for fast switching Schottky rectifiers, high resistivity lubricating and low resistivity lubricating coatings, piezoelectric devices, nonvolatile memories, high temperature wide band gap semiconductors, antireflection coatings, high Tc superconductors, piezoelectric devices and phosphors.
Generally, when using laser deposition techniques, a substrate is coated with a thin film that is generated from a plume of ions and energetic neutral species emanating from a selected target material. A focused pulsed laser beam, usually from a UV source, is incident on the target at a non-perpendicular angle. The deposition is generally performed in a vacuum or selected atmosphere of a reactive gas, such as flowing oxygen or nitrogen. A major advantage of the pulsed laser deposition process is the near stoichiometric transfer from the ablation target to the substrate. This feature is difficult to achieve with other conventional coating technologies. With pulsed laser deposition, minor adjustments to the substrate temperature, and also possibly to the background pressure of reactive gases, result in adherent coatings with acceptable morphologies, correct stoichiometries, and desirable physical properties. Other advantages of laser deposition include a faster deposition rate, the requirement for only a single target, and the ability to deposit materials possessing high boiling point temperatures, such as refractory materials. The ability to select from a wide variety of solid targets with known chemistries and to maintain those chemistries during the coating process, renders laser deposition technologies much more flexible and versatile than other coating technologies.
While pulsed laser deposition offers clear advantages over conventional coating technologies, the current methods and apparatus for practicing pulsed laser deposition have relatively low deposition rates and often introduce macroparticles that have been ejected from the target material. These methods utilize short wavelengths, 193 nm to 308 nm, arriving in pulses 20 to 50 ns wide with peak irradiances of 10.sup.8 to 10.sup.11 W/cm.sup.2 at rates of 5 to 30 Hz and pulse powers in the 10-100 mJ/pulse range. Light at such short wavelengths is difficult to transmit through currently available fiber optics. This limits the flexibility of potential light delivery systems.
Only thin films of very high value are considered economic at present deposition rates which are on the order of about 10 .mu.m-cm.sup.2 /h. This low rate coupled with the high cost of the laser system present a major impediment to the ability to utilize pulsed laser deposition in large scale commercialization applications.
Accordingly, there is a need for a pulsed laser deposition method and apparatus that produces thin films, (i) at much faster deposition rates than current methods with potential wide spread commercial applications, (ii) with a minimal presence of macroparticles from the target material, (iii) which use visible light, (iv) that use laser sources positioned at remote locations from the deposition chamber, (v) that employ fiber optics, (vi) which are capable of simultaneously illuminating either a large number of individual positions in one or more deposition chambers, (vii) that make provisions for introducing reactive gases at high pressures between laser pulses but that maintain good vacuum conditions when the laser plume is present and (viii) provide convenient methods for rapidly introducing or exposing fresh target material at a rate to minimize system downtime but maximize film quality.